In the present specification, definitions of terms are as follows. An MCF includes a plurality of cores extending along a fiber axis in a common cladding. A coating is provided on the outer peripheral surface of the common cladding. “Fiber axis” means a central axis passing the center of a cross section of the common cladding. In general, in a cross section of the MCF perpendicular to the fiber axis of the MCF (hereinafter referred to as “the cross section”), a core constellation defining a relative positional relation of the cores has at least one of line symmetry and rotational symmetry. “Cladding center” means the center of the common cladding cross section. “Cladding shape” means the shape of the outer periphery of the common cladding in the cross section. “Core group” means a group of cores composed of a plurality of cores. “Core group constellation” means a core constellation of a core group in the cross section. “Core group symmetry” means symmetry of the core group constellation in the cross section. As the core group symmetry there are two types of symmetry, rotational symmetry and line symmetry. “Core group position” means a position of a core group in the common cladding cross section. The core group position is a position of a rotation center of a core group when the core group symmetry is rotational symmetry, or, a position of a symmetry axis when the core group symmetry is only line symmetry. “Separation distance” means a distance of the core group position from the cladding center. The separation distance is a separation distance between the rotation center and the cladding center when the core group symmetry is rotational symmetry, or, the shortest distance between the cladding center and a symmetry axis when the core group symmetry is only line symmetry. When a core group has rotational symmetry and line symmetry with respect to two or more axes, a direction of separation of the core group position from the cladding center is “core group separation direction.” In that case, the core group separation direction is preferably a direction different from the symmetry axes of line symmetry and more preferably an intermediate direction between neighboring symmetry axes. In cases where a core group has one-axis line symmetry and rotational symmetry and in cases where a core group has only rotational symmetry, the core group separation direction does not matter. “Cladding symmetry” is an index indicative of a structural feature of the common cladding including the core group constellation in the cross section and means symmetry of a figure defined by a combination of the core group constellation and the common cladding. The cladding symmetry depends on the core group position in the common cladding cross section when the shape of the outer periphery of the cladding has no feature concerning asymmetry, e.g., when the shape of the outer periphery of the cladding is in line symmetry and rotational symmetry like a circular shape. “Coating symmetry” means symmetry of a cross-sectional figure defined by a combination of the core group constellation, common cladding, and coating. The coating symmetry depends on both of the position of the cladding center and the cladding symmetry in the cross section when the shape of the outer periphery of the coating provided on the outer periphery of the common cladding has no feature concerning asymmetry.
FIGS. 1 to 3 are drawings for explaining a cross-sectional structure of an MCF 101. In the cross-sectional structure shown in FIG. 1, the MCF 101 has one core group (six cores 11 to 16) and a common cladding 20 including the core group. The six cores 11 to 16 are arranged in two rows and three columns and this core group constellation composed of the six cores 11-16 has both of line symmetry and rotational symmetry. When the core group constellation has rotational symmetry, the rotation center of the core group constellation is the core group position and coincides with the cladding center in ordinary MCFs. In this case, it is difficult to identify each of the cores by only a look at the fiber end face.
FIGS. 2 and 3 show identification of the cores 11-16 at the two ends of the MCF 101. The illustrated cladding shape is a circle, but the cladding shape may be a regular polygon. FIG. 3 is identification of the cores 11-16 with 180° rotation of one end of the MCF 101 about the fiber axis, with respect to FIG. 2. As shown in these FIGS. 2 and 3, the identification of the individual cores is different between one end face and the other end face of the MCF 101 though the core group constellation looks identical. Similarly, with a twist of the MCF 101, the identification of the individual cores also becomes different though the core group constellation looks identical. Just as described, in the case of the general MCF having the core group symmetry, the existence of the symmetry makes the identification of the individual cores difficult.
FIG. 4 is a drawing for explaining a cross-sectional structure of an MCF 102. The MCF 102 has a structure including a core identification maker 90 extending along the fiber axis, in the vicinity of the core 13 in the configuration of the MCF 101 shown in FIG. 1. The marker 90 allows identification of the core 13 and thus makes all the cores identifiable.
In the invention disclosed in Patent Literature 1, the cladding shape has only line symmetry, without rotational symmetry. The core group constellation has line symmetry as core group symmetry, but the symmetry axis of the cladding shape is not coincident with the symmetry axis of the core group symmetry. In the invention of Patent Literature 1, therefore, losing both of rotational symmetry and line symmetry as cladding symmetry, it is possible to identify each core at the two ends of the fiber.
Patent Literature 2 discloses the cladding shape with a notch, or, the configuration wherein a dummy core extending along the fiber axis is provided in the cladding. The notch or the dummy core can be used as a marker.
FIG. 5 of Patent Literature 3 discloses the configuration wherein the cladding center lies on the symmetry axis and the core group symmetry includes only line symmetry.